1. Field of the Invention
The present invention relates to an automatic focus control device of a camera, particularly to an automatic focus control device in which a photographic lens is moved in various kinds of modes.
2. Description of the Prior Art
There has been already proposed a focus detection device for use in a camera in which two images are formed by focusing light bundles which come from an object to be focused passing through a first and second areas of a photographic lens and of the camera which have symmetrical relation to an optical axis of the photographic lens each other and the mutual relationship of the two image positions is calculated and the amount and the direction of the defocus of the image from the predetermined focal plane can be obtained. Thus, it can be detected whether the image position is in the front side or in the back side of the predetermined focal plane, in other words, in front focus condition or in rear focus condition. The optical system of the focus detection device mentioned above is shown in FIG. 1, wherein the optical system comprises a condenser lens 6 located on a predetermined in-focus plane at the back of a photographic lens 2 located further behind said focal plane 4 are two re-focus image lenses 8 and 10 behind of the condenser 6 and two image sensors 12 and 14 of CCD light receiving elements on the focal plane of the re-focus image lenses 8 and 10 respectively.
As shown in FIG. 2, the images on image sensors 12, 14 draw near each other close to an optical axis 18 in case of the front focus condition in which the image of the object is focused in the front side of the predetermined focal plane, and the images on the image sensors 12, 14 are respectively focused in the position apart from the optical axis in case of the rear focus condition in which the image of the object is focused in the back side of the predetermined focal plane. In case the image of the object is focused in the focal plane, the distance between two corresponding image points on the image sensors 12, 14 corresponds to a predetermined distance defined by the structure of the optical system of the focus detection device. Accordingly, the focus condition can be theoretically obtained by detecting the distance between the two corresponding image points on the image sensors 12, 14.
In an automatic focus control device of a camera, comprising the focus detection optical system mentioned above, there has been used a microcomputer for performing various camera controls such as the integration of the brightness value of the object by CCD image sensor, the calculation of the focus detection or the calculation of the defocus amount and the defocus direction using the CCD image sensor output, the lens movement corresponding to the calculated defocus amount stopping the lens movement in the in-focus position and the shutter release operation.
When the photographic lens is shifted near the in-focus position, the automatic focus control device repeats the sequential automatic focus control operation consecutively and executes a consecutive automatic focus control so that the in-focus condition can be correctly set finally.
By the way, in the automatic focus control device mentioned above, in case the object is closing to the camera or going far away form the camera, even if the automatic focus control operation is performed causing the photographic lens to be moved to the in-focus position on the basis of the detected defocus condition with one time focus detection, the camera can not be set in the in-focus condition for the object because the object is moving during the automatic focus control operation.
FIG. 3, is a graph showing a relation between the time of the horizontal axis and the defocus amount on the photographic film plane in the camera of the vertical axis. In FIG. 3 a curve 1 shows a change of the actual defocus amount on the photographic film when the object is closing to the camera, and a line m shows the defocus amount obtained by converting the object distance of the photographic lens controlled by the automatic focus control device into the corresponding defocus amount.
Periods of time for taking in the object data in the microcomputer are represented by A to D in the middle of the respective integration times of the image sensor respectively. IN FIG. 3, T.sub.0 denotes a center point of the first integration time and D.sub.0 denotes the defocus amount at the time period T.sub.0. The time interval between T.sub.0 to T.sub.1 is the necessary time for the calculation of the focus detection from the center point T.sub.0 to the end of the integration time. When the movement of the photographic lens is completed, the photographic lens is stopped and the next integration during the time periods T.sub.2 to T.sub.3 and the calculation during the time period T.sub.3 to T.sub.4 are executed. At the time T.sub.2 of stopping of the photographic lens, the object has already moved and the defocus amount D.sub.1 -D.sub.0 has occurred compared with the defocus amount at the time T.sub.0. At the time T.sub.3 the data of the object is taken-in and at the time T.sub.5 the defocus amount (D.sub.2 -D.sub.1) is calculated and the movement of the photographic lens is finished. At this time T.sub.5 the object has already moved and even if the movement of the photographic lens is finished, the defocus amount (D.sub.3 -D.sub.2) is occurred and increases larger than the defocus amount at the time T.sub.2. Similarly in the following, at the time T.sub.8 the defocus amount is (D.sub.5 -D.sub.4), at the time T.sub.11 the defocus amount is (D.sub.7 -D.sub.6) etc. The photographic lens goes far away from the in-focus condition and a delay of the tracking of the photographic lens against the movement of the object occurs in spite of the automatic focus control operation whereby a shutter release operation in the in-focus condition can not be completed.
The tracking delay of the lens under the automatic focus control causes a problem especially in case there is adopted an interchangeable having a long focal length, lens such as a telephoto lens having a slow focusing speed.
The applicant of the present application proposed one way of solving the problem mentioned above. The summary of the way is explained with reference to FIG. 4. Assuming that three defocus amounts DFB, DFC and DFD respectively at respective times of integration I2, I3 and I4 are shown as DFB&lt;DFC&lt;DFD, the next new defocus amount is calculated by adding the difference Z between the defocus amount DFD and the defocus amount DFC, whereby the tracking delay for the moving object can be reduced. In FIG. 4 the defocus amount at the time of stopping the photographic lens or at the time of starting the integration I5 can be reduced from X to Y by adding the difference Z between the two defocus amounts DFC and DFD.
However, in the proposed way mentioned above, only the delay of the defocus correction can be compensated. This means, however, that the proposed way is effective only to prevent an excessive correction of the defocus, therefore there sometimes occurs that the revised defocus amount is not sufficient. Judging from the defocus amount it can be seen whether the speed of the movement of the object on the film plane is fast or slow.